Enzymatically active high-flux selectively gas-permeable membranes

ABSTRACT

An ultra-thin, catalyzed liquid transport medium-based membrane structure fabricated with a porous supporting substrate may be used for separating an object species such as a carbon dioxide object species. Carbon dioxide flux through this membrane structures may be several orders of magnitude higher than traditional polymer membranes with a high selectivity to carbon dioxide. Other gases such as molecular oxygen, molecular hydrogen, and other species including non-gaseous species, for example ionic materials, may be separated using variations to the membrane discussed.

PRIORITY

This application claims priority to provisional U.S. Patent ApplicationSer. No. 61/786,404 filed Mar. 15, 2013, the disclosure of which isherein incorporated by reference in its entirety.

This invention was made with Government support under contract no.DE-AC04-94AL85000 awarded by the U.S. Department of Energy to SandiaNational Labs. The Government has certain rights in the invention.

TECHNICAL FIELD

The present teachings relate generally to the field of selectivelypermeable membranes and, more particularly, to a selectively permeablemembrane that uses enzyme catalysis or another catalytic method toaccelerate the selective permeation process.

BACKGROUND

Carbon dioxide (CO₂) separation and capture is a global topic that isclosely related to energy and the environment. For example: CO₂ capturefrom power plant flue gases would dramatically reduce greenhouse gasesand the resulting deleterious effects; CO₂ extraction from low-gradenatural gas is needed as an energy efficient technique to improve purityof the natural gas to pipeline standards; and captured CO₂ may be usedfor enhanced oil recovery (EOR) processes. CO₂ separation from gases andfluids also has many applications in the areas of medical science,chemical engineering, the petroleum industry and even food industries.

Most commercial CO₂ separation plants use a process referred to as“pressure swing adsorption” (PSA), which is based on chemical absorptionwith a monoethanolamine (MEA) solvent. PSA processes require largecapital equipment investment and consume high amounts of energy neededfor regeneration.

Membrane separation is a compact, energy-efficient, and inexpensivealternative to PSA. Some CO₂ membranes have been developed, whichinclude porous CO₂ membranes based on physical separations such asKnudsen diffusion or molecular sieving, as well as dense CO₂ membranes(e.g. polymer membranes) based on chemical separation such as solubilityand diffusion in the solid state. Porous membranes based on physicalseparations suffer from relatively poor selectivities. Additionally,physical separation depends strongly on the composition of other gaseswithin the CO₂ mixture. Deficiencies of dense CO₂ membranes include avery low CO₂ flux across the membrane because of the small CO₂solubility in the membrane and slow diffusion of CO₂ across themembrane. In general, most current CO₂ membrane technologies are notsufficiently efficient for practical applications.

An improved membrane for the effective separation of CO₂ from a gas orliquid, and its method of formation and use, would be desirable.

SUMMARY

The following presents a simplified summary in order to provide a basicunderstanding of some aspects of one or more embodiments of the presentteachings. This summary is not an extensive overview, nor is it intendedto identify key or critical elements of the present teachings, nor todelineate the scope of the disclosure. Rather, its primary purpose ismerely to present one or more concepts in simplified form as a preludeto the detailed description presented later.

In an embodiment, a membrane structure for moving a gaseous objectspecies from a first region having an object species firstconcentration, through the membrane structure, to a second region havinga object species second concentration different from the firstconcentration, may include a supporting substrate comprising a pluralityof pores therethrough and a nanoporous layer within the plurality ofpores. The nanoporous layer may include a hydrophilic layer and ahydrophobic layer. The membrane structure may further include a liquidtransport medium that resides within the hydrophilic layer, wherein theliquid transport medium includes a liquideous permeation medium and atleast one catalyst within the liquideous permeation medium.

In another embodiment, a method for moving an object species from afirst region having an object species first concentration to a secondregion having an object species second concentration different from thefirst concentration using a membrane structure comprising a supportingsubstrate, may include exposing a gas comprising the object species to aplurality of pores within a first side of the membrane structure,dissolving the object species within a liquid transport medium, whereinthe liquid transport medium is within a nanoporous layer that is withinthe plurality of pores, and the liquid transport medium comprises aliquideous permeation medium and at least one catalyst within theliquideous permeation medium. After dissolving the object species withinthe liquid transport medium, moving the object species from the firstside of the membrane structure to a second side of the membranestructure through the liquid transport medium. The method may furtherinclude releasing the object species from the liquid transport medium,out of the membrane structure, and into the second region.

Another embodiment includes a method for making a membrane structure,where the membrane structure is configured to move an object speciesfrom a first region having an object species first concentration at afirst side of a membrane structure to a second region of having anobject species second concentration different from the firstconcentration. The method may include providing a nanoporous supportingsubstrate comprising a plurality of pores therethrough, coating themembrane structure with a coating comprising at least one of a sol-gel,a nanoporous polymer, and a nanoporous organic-inorganic composite tobridge the plurality of pores with the coating, and drying the coatingto form a hydrophobic nanoporous layer within the plurality of pores.The method may further include treating the first side of the membranestructure to convert a first portion of the hydrophobic nanoporous layerto a hydrophilic nanoporous layer, while a second portion of thehydrophobic nanoporous layer remains hydrophobic, and exposing thenanoporous layer to a liquid transport medium wherein the liquidtransport medium remains within the hydrophilic nanoporous layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentteachings and together with the description, serve to explain theprinciples of the disclosure. In the figures:

FIG. 1 is a schematic cross section depicting a membrane structure inaccordance with an embodiment of the present teachings;

FIG. 2 is a magnified view of the FIG. 1 structure including a transportmedium comprising at least one enzyme within a liquideous permeationmedium.

FIG. 3 is a schematic cross section depicting a membrane structure inaccordance with another embodiment of the present teachings; and

FIG. 4 depicts various reactions involved with a specific exemplaryembodiment of the present teachings.

It should be noted that some details of the FIGS. have been simplifiedand are drawn to facilitate understanding of the present teachingsrather than to maintain strict structural accuracy, detail, and scale.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of thepresent teachings, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

An embodiment of the present teachings includes a new technology formaking an enzyme-catalyzed membrane that is effective for CO₂separation, as well as the resulting membrane and its method of use.

For high selectivity, separation based on a chemical process (e.g., acatalytic process) instead of a physical process (e.g., based ondiffusivity difference or size exclusion/molecular sieving) ispreferred. For a membrane that utilizes a chemical process forseparation, the separation process may involve the following steps: 1)an objective species such as CO₂ will interact with the membrane surfaceat the high concentration side of the membrane and dissolve in theliquideous permeation medium. An example would be CO₂ gas dissolved inan aqueous solution (CO₂(g)

CO₂(aq)). 2) The dissolved objective species will be catalyticallyconverted to dissolve into the membrane; 3) the dissolved species suchas CO₂ will travel across the membrane thickness and arrive at the otherside of the membrane, which is the collecting side to collect purifiedCO₂; 4) at the collecting side of the membrane, the dissolved specieswill be released from the membrane. The speeds of these steps aredifferent and the overall transport speed depends most heavily on theslowest step.

Confining the enzyme and liquideous permeation media in anano-structured membrane rather than a macroscopic structure confers atleast two advantages. First, confinement in a pore with diameterslightly larger than the enzyme can result in enhanced enzyme stabilityand catalytic activity, as demonstrated for other enzymes (see, forexample, Lei, et al., Characterization of Functionalized NanoporousSupports for Protein Confinement, Nanotechnology, 17, 2006, 5531-5538;Lei, et al., Entrapping Enzyme in a Functionalized Nanoporous Support,J. American Chemical Society, Published on the Web 2002). Second,confinement in a pore with diameter slightly larger than the enzyme andwidth large enough to accommodate two or more enzymes can result infaster reaction rates due to assembly of an effective multi-enzymecomplex and substrate channeling. In a multi-enzyme complex, the activesites that catalyze chemical reactions are in close proximity. Substratechanneling occurs when the chemical species produced by the first enzymeare also substrates for the nearby enzyme. The multi-enzymeconfiguration speeds chemical reactions by eliminating the time neededin bulk solution for free diffusion of substrates and enzymes to makerandom encounters.

The carbonic anhydrase enzyme can be used to illustrate the advantagesof a nanostructured membrane. The carbonic anhydrase enzyme isspherical, with diameter of 5 nm. A nano-structured pore of diameterslightly larger than the enzyme (e.g. 6 nm) can stabilize the enzyme'sstructure, especially in the region of the active site. Enhancedstructural stability means that the enzyme may retain catalytic activityfor longer times and under a broader range of conditions (e.g., solutionionic strength, temperature). In addition to a nano-structured porediameter, a pore width that accommodates two or more enzymes (e.g., 10nm) can result in assembly of multiple carbonic anhydrases in closeproximity and enhanced catalytic rates. Carbonic anhydrase enzymescatalyze CO₂ hydration or dehydration depending on which chemicalspecies are present in excess. Excess CO₂ at the membrane surfaceresults in enzyme catalysis of CO₂ hydration and formation of protonsand HCO₃ ⁻. The products of that first reaction diffuse down theirconcentration gradients only a short distance and then encounter thenext enzyme. Excess protons and HCO₃ ⁻ in the vicinity of the secondenzyme are substrates for catalysis of HCO₃ ⁻ dehydration. The secondcatalytic (dehydration) reaction forms CO₂ at the collecting end of themembrane. To summarize, nano-confinement of the enzyme and permeationmedia can result in a more stable enzyme structure that tolerates awider range of solution conditions, and faster transport of the objectspecies through substrate channeling by multi-enzyme complexes.

An embodiment of the present teachings may therefore provide a membranethat transports an object species at a broader range of temperatures, orat specific temperatures, not possible with conventional dry polymermembranes. For example, in an embodiment, the membrane structure 10 mayprovide sufficient transport of the object species at a broadertemperature range of between about 0° C. and 60° C., or between about40° C. and about 60° C., or at 0° C., 20° C., 40° C., or 60° C.

FIG. 1 is a schematic cross section depicting a membrane structure 10according to an embodiment of the present teachings. The membranestructure 10 may be used to transport an object species 12 from a firstregion 13 having a first concentration of the object species to a secondregion 15 having a second concentration of the object species. Inembodiments, the first concentration may be higher than or lower thanthe second concentration. The membrane structure 12 may include asupporting substrate 14 comprising a plurality of pores 16. A first end16A of each pore 16 (i.e., an upper end 16A as depicted in the FIG. 1orientation) may have a first diameter while a second end 16B of eachpore (i.e., a lower end 16B as depicted in FIG. 1), wherein the firstdiameter is the same as, or different from, the second diameter. FIG. 1further depicts a nanoporous layer 18 located within each of theplurality of pores 16. The nanoporous layer 18 may include a hydrophilicportion 18A and a hydrophobic portion 18B, and provides a nanoporousframework within the supporting substrate 14.

FIG. 2 is a magnified view of the upper portion 16A of the membranestructure 10 of FIG. 1. For simplicity of depiction, FIG. 2 shows amagnification of two hydrophilic pores within the hydrophilic region18A. The membrane structure 10 may further include a liquid transportmedium 20 formed within the hydrophilic portion 18A of the nanoporouslayer 18 as depicted in FIG. 2. Because the top portion 18A ishydrophilic and the bottom portion 18B is hydrophobic, the liquidmeniscus is depicted as concave on the top and convex on the bottomassuming the liquid medium to be water. The liquid transport medium 20may include a liquideous permeation medium 22, all of which confines anenzyme 24 within the hydrophilic nanoporous region 16A×18A.

In an embodiment, the supporting substrate 14 may be an anodized porousalumina having a thickness of between about 5 nanometers (nm) and about10 millimeters (mm), or between about 5 nm and about 100 micrometers(μm), or between about 1 μm and about 50 μm. The supporting substrate 14may further have an average pore size of between about 10 nanometers(nm) and about 5,000 μm, for example about 20 nm. In an embodiment, thesupporting substrate 14 may be an Anodisc™, available from Whatman plc.Various porous or permeable materials may be sufficient for thesupporting substrate 14, including silica, other oxides or ceramics,polymeric (polymer) materials, other inorganic materials,organic-inorganic composite materials, metallic (metal or metal alloy)materials, metallic composite materials (a metal and a non-metal), or acombination of two or more of these. In an embodiment, the disc may havea width/length or diameter of between about 0.5 centimeters (cm) andabout 1000 cm, or another suitable size depending on the use. The discmay have a round, oval, square, rectangular, folded, pleated, orcorrugated shape, or any number of other shapes depending on use.

At a first concentration side 13 of the membrane structure 10 (i.e., thetop as depicted in FIG. 1), the pores 16A may have an average diameterof between about 2 nm and about 1000 nm, or between about 2 nm and about10 nm, for example about 6 nm. At the second concentration side 15 ofthe membrane structure 10 (i.e., the bottom as depicted in FIG. 1), thepores 14 may have an average diameter of between about 2 nm and about 10mm, or between about 10 nm and about 200 nm, for example about 150 nm,or between about 10 μm and about 10 mm. Thus the pores 16 within thesupporting substrate 14 may or may not include a hierarchic porousstructure along the thickness of the disc, and along a length of thepores 16, such that the pore opening 16A at the top of the membranestructure 10 has a different average diameter than the pore opening 16Bat the bottom of the membrane structure 10. For the case of hierarchicporous structure, the average pore size at the first concentration side13, for example the high concentration side, of the membrane structure10 is smaller than the average pore size at the second concentrationside 15, for example the low concentration side, of the membranestructure 10.

The nanoporous layer 18 may include a mesoporous or nanoporous silicalayer embedded within the pores 16 of the supporting substrate 14. Forpurposes of this application, a “nanoporous” layer is a porous layerhaving an average pore size that is nanoporous or smaller (i.e.,nanoporous or mesoporous), for example, having an average pore size of100 nm or less. In an embodiment, an average size of the nanoporesextending through the nanoporous layer may be between about 6 nm andabout 8 nm in diameter. Various materials other than silica may besufficient for the nanoporous layer 18, including a nanoporous polymericmaterial (i.e., a nanoporous polymer), nanoporous organic-inorganiccomposite, oxides, metals and metal alloys, carbon including grapheneand graphene oxide, sulfides including molybdenum disulfide (MoS₂), andcomposites thereof. The nanoporous layer 18 may be much thinner than thesupporting substrate 14 itself, for example between about 5 nm and 100μm.

The liquideous permeation medium 22 that forms the liquid transportmedium 20 may include (but is not limited to) water, water with salts,water with buffering species that assist in maintaining a constant pH,and combinations thereof, or other nonaqueous solutions.

The enzymes 24 within the liquid transport medium 20 including theliquideous permeation medium 22 may include (but is not limited to)carbonic anhydrases (CAs), or any other enzyme or other catalyst thatcatalyzes the dissolving or releasing process of the “object species” tobe separated by this membrane.

In an embodiment for forming the membrane structure 10, the disc usedfor the supporting substrate 14 may be coated with a coating includingat least one of a sol-gel solution, a nanoporous polymer, or ananoporous organic-inorganic composite. The coating may include asurfactant. The coating bridges the pores 16 within the supportingsubstrate 14 and forms a mesoporous or nanoporous layer 18, for examplea nanoporous silica layer 18 having a thickness of between about 5 nmand about 100 μm as depicted in FIG. 1. The nanoporous silica layer 18may be formed, for example, using a process called “evaporation inducedself-assembly” (see Brinker et al., Advanced Materials, V. 11, no. 7, p.579, 1999). Due to capillarity, the sol-gel solution tends to remainwithin the narrower, upper portion of the pores 16A within thesupporting substrate 14, and does not remain within the wider, lowerportion of the pores 16B after drying. The wider pore portions therebyremain generally free from the material that forms the nanoporous layer.Therefore, the smaller portions 16A of the pores 16 at the top surfaceof the supporting substrate 14 (referring to the orientation of FIGS. 1and 2) were filled with nanoporous silica while the larger portions 16Bof the pores 16 at the bottom surface of the supporting substrate 14were generally left unfilled by the nanoporous silica as depicted inFIG. 1.

After forming the nanoporous layer 18, the sample, for example theentire sample including pores 16A, 16B within the supporting substrate14 and the pores within the nanoporous layer 18, may be treated with ahydrophobic surface treatment, such as an exposure of exposed surfacesto hexamethyldisiloxane (HMDS). This renders both the exposed poroussurface of the nanoporous layer 18 and the surface 26 of the supportingsubstrate 14 hydrophobic. In another embodiment, various structures maybe inherently hydrophobic such that this hydrophobic surface treatmentis optional.

Subsequently, the top surface of the supporting substrate 14 andnanoporous layer 18 were irradiated with an oxygen plasma and ozone. Inan embodiment, irradiation may include an exposure to an oxygen plasmafor a duration of between about 1 second and about 60 seconds, forexample about 5 seconds. The oxygen plasma may be generated by argon(Ar) and molecular oxygen (O₂) in a ratio of between about 0:1 and about100:1, for example about 1:1. The surface treatment may be performed ata vacuum pressure of between about 20 mTorr and about 500 mTorr, forexample about 150 mTorr, and a radiofrequency (RF) power of betweenabout 20 Watts and about 300 Watts, for example about 60 Watts. Theirradiation treatment resulted in a thin portion 18A of the nanoporouslayer 18 becoming highly hydrophilic. In an embodiment, the hydrophilicportion 18A may have a thickness of between about 2 nm and about 10 μm,or between about 2 nm and 1000 nm, or less than about 100 nm, or lessthan about 50 nm. Portion 18B of the nanoporous layer 18 remainshydrophobic. In an embodiment, hydrophobic portion 18B may have athickness of between about 0 nm and about 5 mm.

After the irradiation treatment, the sample was soaked in a volume ofthe liquid transport medium 20, which included the liquideous permeationmedium 22 and the enzyme 24. As most parts of the sample disc, includingportion 18B of the nanoporous layer 18 are inherently hydrophobic withonly the thin top layer 18A being hydrophilic through exposure to theplasma, the transport medium will remain only within the hydrophilicportion 18A of the pores 18. The thickness of this liquideous membranedepends on the time and intensity for plasma and ozone irradiation.FIGS. 1 and 2 depict a schematic cross section of an enzyme-catalyzedmembrane structure according to an embodiment of the present teachings.

This membrane structure 10 has substantial advantages during separationof the object species 12, for example during interaction of the membranesurface on the first concentration side 13, transport of the objectspecies 12 across the membrane structure 10, and release of the objectspecies 12 at the second concentration side 15 as described below. Thegas 28 without the object species 12, or with a lower concentration ofthe object species 12, remains on the first concentration side 13 of themembrane structure 10.

For movement of an object species 12 into the membrane 10, an objectspecies such as CO₂ molecules from the feeding mixture exterior to themembrane structure at the concentration side 13 are dissolved into thetransport medium 20 that includes the liquideous permeation medium 22and the enzyme 24 within the nanoporous layer 18. A liquideouspermeation medium 22 by itself can dissolve CO₂ much faster thansolideous polymers. In addition, this dissolving process can be furtheraccelerated by the CO₂ enzyme 24 embedded in the membrane structure by10⁵ times (see, for example, J. Gutknecht et al., Journal of GeneralPhysiology, V. 69, p. 779, 1977). Dissolution involves hydration(CO₂(aq)) and chemical reaction with a deprotonated water molecule (OH⁻)to form highly soluble species: bicarbonate (HCO₃ ⁻) and a proton (H⁺).Because the membrane structure 10 includes the liquideous permeationmedium 22, step one for this membrane structure 10 is expected to bemuch faster than conventional polymer membranes.

Next, the CO₂ 12 dissolved within the transport medium 20 crosses thethickness of the membrane. The transport medium 20 includes thepermeation medium 22 in liquid phase. In general, the diffusion of theobject species 12 through liquid phase is much faster than the diffusionof the object species through a solid phase alone. For at least thisreason, the dissolved CO₂ can travel from one side of the membrane tothe other side much faster than it can travel through a conventionalpolymer membrane. Further, the movement of the object species across themembrane structure 10 is a catalyzed transport process, because anenzyme accelerates CO₂ dissolution into and release from the transportmedia. In particular, the carbonic anhydrase enzyme catalyzes CO₂dissolution in three steps as depicted in FIG. 4. First, a watermolecule bound to the metal center of the enzyme loses its proton. Thatstep is believed to be rate-limiting. Second, the proton exits tosolution by Grotthus shuttling and the remaining zinc-bound hydroxide(OH⁻) executes nucleophilic attack on a nearby CO₂ to form HCO₃ ⁻. Theenzyme stabilizes CO₂ in the vicinity of the hydroxide to facilitatethat reaction. In the final step, a water replaces HCO₃ ⁻ and the activesite recovers its original form. The result of the enzyme catalysisreaction is enhanced solubility of CO₂ specifically and at fast rates.The enzyme also catalyzes the reverse reaction—dehydration of HCO₃ ⁻specifically to form CO₂ and water at fast rates (counter-clockwise inFIG. 4). Which reaction is favored depends on the availability of CO₂compared to HCO₃ ⁻ and protons in the permeation medium. For example,CO₂ uptake and dissolution is favored at the feeding side of themembrane where CO₂ concentrations are high. HCO₃ ⁻ dehydration and CO₂release to gas phase is favored at the collecting side of the membranewhen the availability of protons and HCO₃ ⁻ is high. The reaction may becatalyzed by one enzyme, or two enzymes, or more than two enzymes, orone or more other catalyst.

In the specific example of carbon dioxide as depicted in FIG. 4, thechemical reaction is catalyzed by an enzyme (En) that converts carbondioxide into the more soluble bicarbonate ion and a proton afterreaction with a metal-bound water: CO₂+H₂O

HCO₃+H⁺. The enzyme may be carbonic anhydrase (CA) with a zinc (Zn²⁺)ion in the active site. The forward reaction is favored in solutionswith excess CO₂. The reverse reaction is favored in solutions withexcess H+ and HCO₃ ⁻.

While the object species 12 will generally move from a region ofrelatively higher object species concentration to a region of relativelylower object species concentration, various conditions may be controlledso that the object species is pumped from a region of lowerconcentration to a region of higher concentration. For example, theatmospheric pressure and/or atmospheric temperature may be increasedwithin the region of relatively lower concentration to move the objectspecies toward the region of relatively higher concentration. Also, theenzyme may be modified to change its catalytic behavior. The electronicenvironment around the metal ion enhances the acidity of the metal-boundwater to a value of 6.8 in units of pK_(a) compared to the value of 15.7in liquid water. Thus the metal-bound water tends to lose its protoneven in neutral solutions (pH=7), meaning that the enzyme favors uptakeof CO₂ in neutral solutions. Mutations to amino acid residuessurrounding the metal ion can alter the electronic environment andchange the acidity of the metal-bound water, and thus change whether CO₂uptake or release is favored for specific solution conditions.

As described above, the nanoporous layer (e.g., the nanoporous silica)18 within the pores 16 may be treated so that a first portion 18A, theportion adjacent to the high CO₂ concentration region 13, is hydrophilicwhile a second portion 18B, the portion away from the high CO₂concentration region and closer to the lower CO₂ concentration region15, is hydrophobic. The hydrophilic region 18A may thus be referred toas an active layer of the nanoporous layer 18 and the hydrophobic region18B may be referred to as a passive layer of the nanoporous layer 18.The liquid transport medium 20 is thus drawn toward the hydrophilicregion 18A such that the hydrophobic portion 18B of the nanoporous layer18 remains dry or dryer than the hydrophilic portion 18A. The net effectis a thinner layer of the transport medium 20. Because the transportmedium 20 is thinner, resulting from the thin hydrophilic region 18A,the transport time across the transport medium 20 is decreased as thedistance across the transport medium 20 is decreased. The thickness ofthe active layer 18A may be very small as long as the nanoporous layer18 is sufficiently thin.

Forming a modified (e.g., hydrophilic) silica layer that has a totalthickness of 10 nm or less using plasma-assisted atomic layer deposition(ALD) process has been demonstrated (see Jiang et al., J. AmericanChemical Society, 2008). A silica layer having a thickness of less than10 nm thick can be formed with a plasma-assisted ALD process asdescribed below to form a nanoporous layer 18 of a membrane structure 10to filter an object species, such as a CO₂ membrane structure. Inaddition, the thickness of the active layer 18A, i.e., the hydrophilicportion of the silica layer, may be controlled by partially modifyingthe exposed surfaces of the nanoporous silica to be either hydrophilicor hydrophobic, as discussed above. In an embodiment, the layer ofliquid transport medium 20 within the nanoporous silica 18 may thus havea thickness of between about 2 nm and about 10 μm, or between about 2 nmand about 10 nm.

The object species 12 that is dissolved within the transport medium 20is released therefrom on the low concentration side 15 (i.e., thecollecting side) of the membrane structure 10. The release process maybe catalyzed by another enzyme or the same enzyme 24 as used in step oneto increase the releasing rate. The CA enzyme may be used to bothcatalyze the CO₂ dissolving step and to catalyze the CO₂ releasing step.During transport through the transport medium 20, it will be noted thatthe CO₂ may not remain as molecular CO₂.

CO₂ permeance and selectivity measurements were carried out at roomtemperature. To ensure that the liquid transport medium 20 did not dryout, a water bubble generator was used. The feed gas, including theobject species 12 and the carrier gas 28, was humidified by passing thefeed gas through a water bubble generator so that the feed gas carried ahigh concentration of water vapor. To increase the water vaporconcentration, the bubble generator may be heated to increase the watervapor carrying capacity of the feed gas. The water vapor will condenseinside the hydrophilic portion 18A of the nanoporous layer 18 becauseof, for example, capillary condensation. However, the hydrophobicportion 18B of the nanoporous layer 18 remains empty or mostly emptybecause little or no capillary condensation takes place. The liquideoustransport medium 20 of the membrane structure 10 within the hydrophilicportion 18A of the nanoporous layer 18 thus remains very stable duringthe separation of the object species 12 from the feed gas 12, 28 usingthe membrane structure 10 as a filter. A typical CO₂ flux was 0.2 cc persquare centimeter per minute (cm³/cm²/min) at 1 atmosphere (atm) ofpressure difference for the produced membranes. The highest flux was 1.2cm³/cm²/min at 1 atm pressure difference. The membrane may provide a CO₂flux of 0.4-2.5(10)⁶ Barrer (1 Barrer=10⁻¹⁰ cm³/cm⁻²-s-cmHg⁻¹ atstandard temperature and pressure, STP). When argon gas was used for themeasurement, no permeance was detectable after 30 minutes. The combinedresults indicate a high flux and a high, almost perfect selectivity ofobject species by this membrane. In embodiments, the ratio of CO₂ toargon fluxes (i.e., the selectivity of CO₂ compared to argon duringtransport) may be greater than 500:1.

For a dense ultra-thin membrane structure 10, one limiting step for theflux through the membrane structure 10 is typically the initial reactionof the object species 12 into the membrane surface at the highconcentration side 13 of the membrane structure 10. In many cases, thetemperature of the feed gas and/or the membrane surface may be increasedto catalyze the surface reaction rate. However, this heating processresults in negative aspects such as increased energy consumption,high-temperature sealing of the membrane pores 16, etc. In an embodimentof the present teachings, enzymes were used to catalyze the surfacereaction steps. The reaction rate may be increased by more than onehundred thousand times while maintaining the membrane structure 10 atroom temperature. In addition to enzyme catalysis through the use of anenzyme 24 within the liquideous permeation medium 22, other catalyzationprocesses, such as photo-catalyzation, catalyzation by using catalysis,etc., may be used.

The transport medium 20, which is the selectively permeable component ofthe membrane structure 10, may be a thin layer of aqueous or otherliquid solution. This is advantageous over traditional dense (i.e.,solid) polymer membranes in both the solubility of the object species tobe separated and the diffusivity for the species to travel across themembrane thickness. The permeation medium 22 of the liquid transportmedium 20 in accordance with an embodiment of the present teachings maybe in one or more of various liquideous forms, e.g., a liquideous formwith one or more other solvents such as one or more organic solvents, orin a gelatinous (i.e., gel-like) form.

The thickness of the membrane structure 10 may be reduced by using athinner starting nanoporous film 18 during the manufacturing process.Additionally, the thickness of the membrane structure 10 may be reducedas described above by partial modification of the nanopore surfacechemistry so that only a thin layer 18A of the nanoporous layer 18 hasthe property of holding the liquid transport medium 20. In other words,a partial thickness of the supporting substrate 14 and the nanoporouslayer 18 may be converted from a hydrophobic surface to a hydrophilicsurface, while a partial thickness of the membrane structure remainshydrophobic, thus allowing a reduction in the thickness of the activelayer 18A compared to the overall thickness of the membrane structure10. This modification may be achieved by plasma treatment as describedabove, or by using an ozone treatment, or using other methods or acombination of methods.

A membrane structure 10 in accordance with the present teachings may beused for separation of CO₂ from a gaseous medium. Membrane structuresmay be formed to separate other object gases, for example, molecularoxygen (02), molecular hydrogen (H₂), or other species includingnon-gaseous species such as ions.

In an embodiment, the transport medium 20, including the liquideouspermeation medium 22 and the enzyme catalyst 24 may be encapsulated byanother semi-permeable layer, e.g., a lipid bi-layer or another porouslayer where the pore size is smaller than the pore size in the activelayer. Also, this liquid transport medium 20 may be sandwiched betweentwo porous layers to improve its mechanical property, for example ascratch resistance of the membrane structure 10 during contact withother structures. The surface chemistry of the nanoporous layer 18 maybe modified using other techniques not described herein to enhancevarious other membrane performance characteristics.

In another embodiment, the hydrophilic portion 18A of nanoporous layer18 may be sandwiched between two nanoporous hydrophobic portions 18B,18C as depicted in FIG. 3. Forming layer 18A between two nanoporoushydrophobic portions 18B, 18C may provide improved mechanicalproperties, for example improved scratch resistance. To form layer 18C,a structure similar to the FIG. 1 structure may be formed, then theupper portion of 18A may be treated to convert it to a hydrophobicregion. For example, the upper surface of hydrophilic region 18A in FIG.1 may be exposed to HMDS to convert the hydrophilic upper portion 18A ofnanoporous layer 18 to hydrophobic portion 18C as depicted in FIG. 3.

Without intending to be bound by theory, it is believed that thedifference in gas concentration between the first concentration side 13and the second concentration side 15 is a driving force for the objectspecies to move from one side of the membrane to the other, particularlywhen moving the object species 12 from a region of relatively higherobject species concentration to a region of relatively lower objectspecies concentration. Considering a CO₂ object species, the CO₂dissolving process is a reversible process in which the CO₂ may bedissolved into the liquid transport medium 20, and may also be releasedfrom the solution 20. Higher CO₂ concentrations on the highconcentration side 13 favors rapid dissolving of the object species 12into the solution 20 and slower release from the solution 20 back intothe higher concentration side 13. Contrarily, as the gas permeates theliquid transport medium 20, lower CO₂ concentrations on the lowconcentration side 15 favors rapid release of the object species 12 fromthe solution 20 into the lower concentration side 13 and slowerdissolving of the gas back into the solution 20. This pump results inoverall net CO₂ flow from the region of higher gas concentration 13 tothe region of lower gas concentration 15. For the reversibledissolving/releasing process, a balance between the dissolving and thereleasing steps will be established. At a certain CO₂ pressure in thefeed gas 12, 28, the dissolved CO₂ concentration is a constant. A higherCO₂ pressure results in a higher concentration of CO₂ dissolved withinthe solution 20. Therefore, the dissolved CO₂ has a higher concentrationat the feeding side 13 (for example the high concentration side) thanthe collecting side 15 (for example the low concentration side). For atleast this reason, the dissolved CO₂ will move from the highconcentration side to the low concentration side through the separationlayer catalyzed by the enzyme, for example CA.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the present teachings are approximations, thenumerical values set forth in the specific examples are reported asprecisely as possible. Any numerical value, however, inherently containscertain errors necessarily resulting from the standard deviation foundin their respective testing measurements. Moreover, all ranges disclosedherein are to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5. In certain cases, the numerical values asstated for the parameter can take on negative values. In this case, theexample value of range stated as “less than 10” can assume negativevalues, e.g. −1, −2, −3, −10, −20, −30, etc.

While the present teachings have been illustrated with respect to one ormore implementations, alterations and/or modifications can be made tothe illustrated examples without departing from the spirit and scope ofthe appended claims. For example, it will be appreciated that while theprocess is described as a series of acts or events, the presentteachings are not limited by the ordering of such acts or events. Someacts may occur in different orders and/or concurrently with other actsor events apart from those described herein. Also, not all processstages may be required to implement a methodology in accordance with oneor more aspects or embodiments of the present teachings. It will beappreciated that structural components and/or processing stages can beadded or existing structural components and/or processing stages can beremoved or modified. Further, one or more of the acts depicted hereinmay be carried out in one or more separate acts and/or phases.Furthermore, to the extent that the terms “including,” “includes,”“having,” “has,” “with,” or variants thereof are used in either thedetailed description and the claims, such terms are intended to beinclusive in a manner similar to the term “comprising.” The term “atleast one of” is used to mean one or more of the listed items can beselected. Further, in the discussion and claims herein, the term “on”used with respect to two materials, one “on” the other, means at leastsome contact between the materials, while “over” means the materials arein proximity, but possibly with one or more additional interveningmaterials such that contact is possible but not required. Neither “on”nor “over” implies any directionality as used herein. The term“conformal” describes a coating material in which angles of theunderlying material are preserved by the conformal material. The term“about” indicates that the value listed may be somewhat altered, as longas the alteration does not result in nonconformance of the process orstructure to the illustrated embodiment. Finally, “exemplary” indicatesthe description is used as an example, rather than implying that it isan ideal. Other embodiments of the present teachings will be apparent tothose skilled in the art from consideration of the specification andpractice of the disclosure herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the present teachings being indicated by the following claims.

Terms of relative position as used in this application are defined basedon a plane parallel to the conventional plane or working surface of aworkpiece, regardless of the orientation of the workpiece. The term“horizontal” or “lateral” as used in this application is defined as aplane parallel to the conventional plane or working surface of aworkpiece, regardless of the orientation of the workpiece. The term“vertical” refers to a direction perpendicular to the horizontal. Termssuch as “on,” “side” (as in “sidewall”), “higher,” “lower,” “over,”“top,” and “under” are defined with respect to the conventional plane orworking surface being on the top surface of the workpiece, regardless ofthe orientation of the workpiece.

The invention claimed is:
 1. A membrane structure for moving an objectspecies from a first region having an object species firstconcentration, through the membrane structure, to a second region havinga object species second concentration different from the firstconcentration, wherein the object species is a gaseous molecule ornon-gas species to be selectively transported from the first region tothe second region, the membrane structure comprising: a supportingsubstrate comprising a plurality of pores therethrough; a nanoporouslayer within the plurality of pores, the nanoporous layer being thinnerthan the supporting substrate, wherein the nanoporous layer comprises: ahydrophilic layer having a thickness of 50 nm or less; and a hydrophobiclayer; and a liquid transport medium within the hydrophilic layer,wherein the liquid transport medium comprises: a liquideous permeationmedium; and at least one catalyst within the liquideous permeationmedium.
 2. The membrane structure of claim 1, wherein: the plurality ofpores have a first portion having a first average diameter at a firstside of the supporting substrate adjacent to the first region; theplurality of pores have a second portion having a second averagediameter at a second side of the supporting substrate adjacent to thesecond region, wherein the first average diameter is smaller than thesecond average diameter; and the nanoporous layer is within the firstportion of the plurality of pores.
 3. The membrane structure of claim 2,wherein the second portion of the plurality of pores is free from thenanoporous layer.
 4. The membrane structure of claim 2, wherein: thefirst average diameter is between about 2 nanometers (nm) and about 1000nm; and the second average diameter is between 0.5 μm and about 10millimeters.
 5. The membrane structure of claim 1, wherein thesupporting substrate comprises at least one of anodized porous alumina,silica, ceramic, a polymer, a metal, a metal alloy, a metallic compositematerial, and combinations of two or more of these.
 6. The membranestructure of claim 1, wherein the nanoporous layer comprises at leastone of silica, silicate, a metal oxide, or a combination thereof.
 7. Themembrane structure of claim 1, wherein the liquideous permeation mediumcomprises water and the catalyst is an enzyme comprising carbonicanhydrase.
 8. The membrane structure of claim 7, wherein the membranestructure is configured to move an object species comprising carbondioxide.
 9. The membrane structure of claim 1, wherein the nanoporouslayer has a thickness of between about 5 nanometers and about 100micrometers and the hydrophilic layer has a thickness of about 2nanometers to about 10 nanometers.
 10. The membrane structure of claim8, wherein the supporting substrate has a thickness of between about 10micrometers and about 200 micrometers.
 11. The membrane structure ofclaim 1, further comprising: the plurality of pores have a first portionhaving a first average diameter at a first side of the supportingsubstrate; the supporting substrate comprises at least one of anodizedporous alumina, silica, ceramic, a polymer, a metal, a metal alloy, ametallic composite material, and combinations of two or more of these;the plurality of pores have a second portion having a second averagediameter at a second side of the supporting substrate, wherein the firstaverage diameter is smaller than the second average diameter; thenanoporous layer comprises at least one of a silica, silicate, metaloxide, or a combination thereof and is within the first portion of theplurality of pores; the second portion of the plurality of pores is freefrom the nanoporous layer; and the liquideous permeation mediumcomprises water and the catalyst is an enzyme comprising carbonicanhydrase.
 12. A membrane structure for moving an object species from afirst region having an object species first concentration, through themembrane structure, to a second region having an object species secondconcentration different from the first concentration, wherein the objectspecies is a gaseous molecule or non-gas species to be selectivelytransported from the first region to the second region, the membranestructure comprising: a supporting substrate comprising a plurality ofpores therethrough; a nanoporous layer within the plurality of pores,the nanoporous layer being thinner than the supporting substrate,wherein the nanoporous layer comprises: a hydrophilic layer having athickness of 50 nm or less and being configured to allow a liquidtransport medium to be positioned within the first layer; and ahydrophobic layer.
 13. The membrane structure of claim 12, wherein thesupporting substrate comprises at least one of anodized porous alumina,silica, ceramic, a polymer, a metal, a metal alloy, a metallic compositematerial, and combinations of two or more of these.
 14. The membranestructure of claim 13, wherein the nanoporous layer comprises at leastone of nanoporous silica, mesoporous silica, nanoporous polymericmaterial, nanoporous organic-inorganic composite, silicates, oxides,metals, metal alloys, carbon, sulfides and composites thereof.
 15. Themembrane structure of claim 12, wherein the supporting substratecomprises at least one of anodized porous alumina, ceramic, a polymer, ametal, a metal alloy, a metallic composite material, and combinations oftwo or more of these.
 16. The membrane structure of claim 15, whereinthe nanoporous layer comprises at least one of nanoporous silica,mesoporous silica, silicate, a metal oxide, or a combination thereof.17. The membrane structure of claim 12, wherein the nanoporous layercomprises nanoporous silica or mesoporous silica.
 18. The membranestructure of claim 15, wherein the supporting substrate comprisesanodized porous alumina.
 19. The membrane structure of claim 15, whereinthe supporting substrate comprises a polymer.
 20. The membrane structureof claim 12, wherein the hydrophilic layer has a thickness of about 2nanometers to about 10 nanometers.